1. Field of the Invention
The present invention relates to an image reader or an image reading apparatus for photoelectrically reading an original image.
2. Related Background Art
To photoelectrically read an original image in digital copying machines and facsimile machines, there have been employed image readers provided with photoelectric transducers such as CCDs. This type image reader includes a processing function, called shading correction, for evenly reading image density.
Heretofore, the shading correction is carried out by placing a standard white plate with known density outside an original read area, reading the standard white plate prior to reading an original, and storing a read signal in a first memory. Based on the data thus stored, the shading correction is effected for the read data resulted from reading the original.
Generally, a reader is arranged to make main scan reading of the document by using a one-dimensional CCD, and mechanically moving a read line of the CCD in a direction perpendicular to the direction of main scan. The standard white plate is placed at a position from which reading in the direction sub scan is started.
In the shading correction, a multiplication coefficient for converting the light intensity data, stored in the first memory and obtained from the standard white plate for each pixel, into a light intensity value corresponding to a known intensity value of the standard white plate, followed by storing the light intensity data in a second memory.
Then, when reading the original, the multiplication coefficient corresponding to a pixel being read is read out of the second memory and multiplied by a read signal of the original to effect the shading correction.
In the above prior art, because an original is placed on a platen glass, the original read signal is a signal having passed through the platen glass. On the contrary, because a standard white plate is fabricated by being painted on a metal thin sheet, it is not always arranged at the same height as the original surface and positioned at a level corresponding to the underside of the platen glass in many cases. In particular, where an automatic original feeder is mounted over the original platen glass, the standard white plate cannot be disposed on the platen glass as, if so, it would impede feeding of originals. Accordingly, the standard white plate is provided on the side of the platen glass opposite to the original surface, i.e., inside the reader.
While an original is covered by the platen glass, the reading surface of the standard white plate, prepared by painting, is directly bared to a light source and a CCD sensor. During use over a long term, therefore, the standard white plate directly exposed to an illumination is remarkably discolored in comparison with the document surface, thus resulting in a large deviation from the known density value initially set.
The standard white plate is also stained with deposition of dust and oil vapor inside the reader.
Since the shading correction is based on an assumption that the standard white plate has known density, the original read signal after the shading correction is inevitably deviated from a predetermined value due to changes in density of the standard white plate make. As a result, the original read signal may be varied to make density lighter or deeper, or in a color reader, color decomposition signals of R, G, B are ill balanced so that color tones and gray balance are no longer ensured.
As a known technique to compensate density changes of the standard white plate, there is proposed U.S. Pat. No. 4,888,492. With this technique, a totally white original is placed as a reference on the platen glass and density changes of the standard white plate are compensated on the basis of read data from the totally white original.
However, because such compensation is usually carried out by a serviceman who manually put a sheet of totally white paper on the platen glass, the above known technique has suffered from the disadvantage that the operation is troublesome and read quality may deteriorate if density of the standard white plate is changed before routine visit of the serviceman or before implementing the operation.